The present invention concerns a position detector system including a transmitter or sending unit consisting of a circuitous conductor. Such position detecting systems are already known and are used for example accurately to position magnetic read and write heads in relation to desired disk-memory tracks.
In the known detectors, the sending means consists usually of a flat continuous conductor arranged in a Greek key pattern presenting transverse, rectilinear parallel sections which are equidistantly spaced and are connected end-to-end at alternate sides of the center line of the pattern by longitudinal conductor sections. Such a conductor, best produced in the form of a printed circuit, is used to carry an alternating current at a frequency of for example 1 MHz, in order to produce an essentially regular pattern of magnetic fields. A pick-up means in the form of at least one magnetic receiver head is moved along a path parallel to the center line of the pattern so as to produce alternate nodes and antinodes at regular intervals dependent upon the disposition of the head relative to the legs of the pattern. More specifically, the signal which the pick-up means receives is modulated and forms a sequence of antinodes when it is perpendicular to the transverse sections and nodes when it is perpendicular to the spaces between said transverse sections. Suitable electronic means may be employed for utilizing a part of the envelope of this modulated signal to count its maxima (perpendicular to the transverse sections) and/or, most of all, the passages through zero (perpendicular to the spaces between said transverse sections). Thus due to this counting, it is possible to sense the number of transverse sections which pass under the receiving head thereby to ascertain the position of the pick-up means relative to the sending unit.
Prior art system ssuch as described above are subject, however, to certain disadvantages. In the first place, unless the transverse rectilinear sections of the Greek key are of considerable length which correspondingly increases the width of that Greek key, the receiving head is sensitive not only to the signal emitted by the transverse sections, but also to those emitted by the longitudinal sections. Consequently, the entire signal is disturbed and is no longer a regular sequence of envelopes and nodes.
A known technique is used by which it is relatively easy to avoid this first pitfall. Since the two groups of discontinuous longitudinal sections of the Greek key which are disposed at opposite sides of the pattern each have a total length equal to half the length of the Greek key, and since each carries the same current I as the transverse sections, the magnetic fields created by each group may be effectively compensated or neutralized by returning one-half the current along a conductor having a length equal to that of the Greek key and parallel to the longitudinal sections. Such a technique carried out in a known manner by establishing a return-path conductor around the Greek key in the form of a rectangular return conductor whose two sides are parallel to the transverse sections of said Greek key. Since the other two sides of the return-path conductor are parallel to the opposite end legs or longitudinal sections of the pattern, an electrical connection is established between one end leg of the Greek key and that side of the return-path conductor frame which is adjoining and parallel thereto. This connection is accomplished symmetrically with respect to the pattern by a longitudinal conductor whose axis is coincident with the longitudinal axis of the Greek key so that the return current splits at such connection and passes in opposite directions therefrom. In this way, the two return runs of the current having traversed the Greek key are identical lengths so that the current is split in half.
Another drawback of the aforesaid system stems from the fact that the amplitudes of the antinodes and the clear formation of the nodes are not uniform throughout the length of the pattern. That is to say, when the pick-up means is located in the central region of the pattern, it receives signals not only from the sending unit legs or patterns immediately near it but also from a number of transverse sections on either side thereof. Consequently, the pick-up means senses the resultant of a great number of signals so that the nodes and antinodes are well defined. When the pick-up means is close to an end of the Greek key, however, it receives a lesser number of signals in consequence of which the nodes in particular are not well defined. That is, the symmetry of the signals received is interrupted with the result that the pick-up means senses much more substantial antinodes and much less clear nodes than in the neighborhood of the center of the Greek key. It becomes difficult then, if not impossible, accurately to detect, and thus count, the nodes in the regions of the ends of the pattern.
A solution for remedying this inconvenience would be to provide a Greek key which is markedly longer than there is a need for and to use only the central portion of the pattern. However, this would increase the crowding of the detector.